arm64: dts: imx8mq-librem5: add RGB pwm notification leds

Describe the RGB notification leds on the Librem 5 phone.
Use the common defines so we're sure to adhere to the common patterns,
use predefined led colors and functions so we're being warned in case
of deprecations.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>